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As with most computer-related technologies, disk drives changed over time after their introduction. In particular, they got bigger. Not larger in physical size, but bigger in their capacity to store information. And, this additional capacity drove a fundamental change in the way disk drives were used.
Partitions: Turning One Drive Into Many
Disk drives can be divided into _partitions_. Each partition can be accessed as if it was a separate disk. This is done through the addition of a _partition table_.
There are several reasons for allocating disk space into separate disk partitions, for example:
Logical separation of the operating system data from the user data
Ability to use different file systems
Ability to run multiple operating systems on one machine
While the diagrams in this chapter show the partition table as being separate from the actual disk drive, this is not entirely accurate. In reality, the partition table is stored at the very start of the disk, before any file system or user data. But for clarity, they are separate in our diagrams.
Disk Drive with Partition Table
Image of an unused disk drive with a partition table.
Each partition table entry contains several important characteristics of the partition:
The points on the disk where the partition starts and ends
The partition's type
The type is a number that identifies the partition's anticipated usage. Some operating systems use the partition type to denote a specific file system type, to flag the partition as being associated with a particular operating system, to indicate that the partition contains a bootable operating system, or some combination of the three.
Disk Drive With Single Partition
Image of a disk drive with a single partition.
The single partition in this example is labeled as `DOS`. This label shows the *partition type*, with `DOS` being one of the most common ones. The table below shows a list of some of the commonly used partition types and hexadecimal numbers used to represent them.